There is no rule preventing any aircraft "installing" a
removable device (other than possibly a CAA approved company procedures manual banning such).
Anything that is removable needs no certification or any approval.
So, you can "install" FLARM, a handheld radio, a satellite phone, you name it. All 100% legal (unless emitting on frequencies which are illegal to transmit on in the UK).
The grey area can be in the powering arrangements. If you get a proper power connector installed somewhere, that ought to be signed off, although very often they just "appear" and nobody asks when it appeared
The alternative is to use the cigar lighter socket which is messy at best.